This invention relates to remediation of undesirable emissions from certain common sources. The invention is described in connection with treatment of emissions from heat-treating furnaces used for production of various parts using so-called xe2x80x9cpowder metallurgyxe2x80x9d techniques, but can be used in remediation of emissions from other sources, such as heat treatment of carbon and graphite parts, heat treatment of fabrics, and battery manufacture, among others.
xe2x80x9cPowder metallurgyxe2x80x9d as used herein refers generally to techniques whereby metal parts of relatively complicated shape can be made relatively inexpensively. In a first step, a mold is filled with powders of the constituent materials, mixed with a binder. The filled mold is then commonly subjected to a first low-temperature (e.g., 400xc2x0 F.) heat treatment, curing the binder, so that the xe2x80x9cgreen partxe2x80x9d or xe2x80x9cpreformxe2x80x9d thus formed holds its shape accurately when removed from the mold. The binder materials are chosen so as to also serve a lubricant function, allowing the green part to be readily removed from the mold after this first heat treatment step. The green part is then sintered, that is, subjected to a high temperature heat treatment (e.g., 2300xc2x0 F. for on the order of minutes or tens of minutes), during which the powders bond securely to one another, forming an essentially finished part. As is well known to those of skill in the art, these parameters vary widely; for example, some sintering processes may take up to 60 hours.
It will be apparent that this process can be much less expensive for forming complex parts than doing so by machining, in that there is little or no waste of material, because materials that are difficult to machine can be formed using these techniques, and because the powder metallurgy process involves reduced manpower and equipment requirements. Consequently, the powder metallurgy industry has grown significantly in recent years.
The typical binder/lubricants are waxy hydrocarbons, such as zinc and lithium stearates, and are commonly provided at 0.5-2% by weight of the green part. Other binder materials can include organic compounds of various proprietary and custom-blended hydrocarbon binder materials. One limitation on the selection of the binder materials is due to the fact that if the furnace exhaust is not properly remediated, the binder materials may result in noxious and odiferous fumes; that is, unless the exhaust is successfully remediated, a preferred binder may not be usable. To remediate exhaust fumes resulting from sintering operations, and thus to allow use of essentially any desired binder material, is accordingly one object of the invention.
More specifically, during the first few minutes of the high-temperature sintering step, the binder/lubricant used to maintain the shape of the green part is driven off and partially decomposed. That is, because a reducing atmosphere is provided within the sintering furnace to prevent oxidation of the parts being sintered, the binder/lubricant, although often nominally combustible, does not burn within the furnace. Therefore, as the binder/lubricant is driven off, waxy emissions, consisting largely of unburned hydrocarbons and like combustible pollutants, are given off, often resulting in visible air pollution and noxious odors. The remaining emissions can also condense on the inner walls of the exhaust flue, forming a potential fire hazard. While these pollutants are not currently regulated, the problem, particularly the threat of stack fires, is significant enough that some action needs to be taken.
One known approach to the problem of remediating the furnace exhaust stream is to collect the particulates in a baghouse or another physical filter. This approach is used to collect both combustible binders, such as zinc and lithium stearates, and noncombustible residues, such as zinc and lithium oxides. However, such constructions are prohibitively expensive for the typical small powder metallurgy facility. Furthermore, the exhaust stream must be significantly cooled before introduction into the filter, such that upstream condensation can still occur, with the consequent risk of stack fires. Filters also clog and require regular maintenance.
Another known way to try to remediate furnace emissions is to place an auxiliary natural gas or other burner directly in the exhaust stack, shortly downstream of the furnace, and simply combust the unburned hydrocarbons and other combustible pollutants as they exit the furnace. This does reduce the level of emissions in the stack exhaust. However, this has not been done properly. Specifically, adding significant additional heat from an added burner to a simple uninsulated exhaust stack creates an enormous draft, such that the furnace exhaust tends to be drawn through and out of the stack before the pollutants are fully combusted. The draft induced in such an arrangement can also draw the reducing gas out of the furnace, requiring its replacement and adding cost. Such auxiliary burner arrangements, as they have been used to date, have also been relatively inefficient and consume large quantities of additional fuel, adding significantly to the cost of sintering operations.
It is therefore apparent that the powder metallurgy industry, and related industries, require improved apparatus and methods for remediation of furnace exhaust gases, that is, for significantly reducing or disposing of undesirable combustible gases and pollutants, and other residues of the waxy hydrocarbon materials used as binder/lubricant compositions in sintering operations, and to provide these is a principal object of the invention.
More specifically, it is an object of the invention to provide apparatus and methods for remediation of exhaust streams from heat treatment and other furnaces, so that essentially any desired combustible binder material can be used.
To be commercially acceptable, any such unit should be economical to purchase and operate, that is, simple in construction and efficient in its use of fuel, as well as being easy to install and service, and such is accordingly a further object of the invention.
The present invention satisfies the needs of the art and the objects of the invention mentioned above, and others which will appear as the discussion below proceeds, by providing an auxiliary burner for combusting waxy hydrocarbons and other emissions in the exhaust stream from the furnace, as in the prior art, but optimized for efficient and substantially complete combustion of the waxy hydrocarbon residues emitted as the binder/lubricant is driven off in sintering of green powdered-metal parts. As noted, the same invention has applicability to treatment of exhaust streams from furnaces used in other industries.
According to the invention, measures are taken to ensure that the furnace exhaust remains in a combustion zone of at least a minumum temperature for an average residence time sufficient to ensure substantially complete combustion. In so doing the waxy binder materials are oxidized to significantly lesser amounts of essentially benign materials. For example, zinc stearates that would otherwise typically be emitted are oxidized to carbon dioxide, water, and zinc oxide, an essentially harmless powder. The design of the chamber within which the combustion takes place is also optimized to provide highly efficient combustion. The overall design of the unit is optimized so as to use as many standard components as possible, to reduce cost, and to simplify installation, proper use, and maintenance.
In the embodiment specifically described, the furnace exhaust remediation unit of the invention comprises a vertical combustion chamber of high-quality stainless steel, to resist corrosion in this hostile environment. However, the combustion chamber need not be vertical. The combustion chamber is lined with a thick layer of one or more types of highly efficient and durable insulation, so as to retain the heat of combustion within the combustion chamber, improving efficiency and reducing the consumption of fuel. Several options are provided to control the rate of flow of ambient air and furnace exhaust through the combustion chamber, permitting control of the average residence time of the particulates and other emissions. The auxiliary burner is arranged such that it provides a flame directed tangentially into the combustion chamber, so that a spiral motion is provided to the exhaust stream, ensuring efficient heat transfer between the flame and the emissions to be oxidized.
A standard gas burner of up to 600,000 BTU/hour capacity, essentially as used on domestic heaters and like appliances, is preferably used as the auxiliary burner, complete with its blower, gas control solenoid, pilot or igniter, and control circuit, to reduce initial cost and simplify repair when necessary. A thermocouple may be provided to monitor the temperature within the combustion chamber, to ensure the temperature is high enough to provide substantially complete combustion while not so high as to pose a safety problem. The entire assembly may be mounted on an intake hood secured over the inlet of the furnace, or may be suspended from the ceiling of the facility, and requires only fuel and electrical connection for operation, further simplifying installation.